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A Raman amplifier is an optical amplifier based on Raman gain, which results from the effect of stimulated Raman scattering.
The Raman-active medium is often an optical fiber, although it can also be a bulk crystal, a waveguide in a photonic integrated circuit, or a cell with a gas or liquid medium.
An input signal can be amplified while co- or counterpropagating with a pump beam, the wavelength of which is typically a few tens of nanometers shorter.
For silica fibers, maximum gain is obtained for a frequency offset of ≈ 10–15 THz between pump and signal, depending on the composition of the fiber core.

Raman amplifiers can be operated in very different wavelength regions, provided that a suitable pump source is available.

The gain spectrum can be tailored by using different pump wavelengths simultaneously.

A Raman amplifier requires high pump power (possibly raising laser safety issues) and high pump brightness, but it can also generate high output powers.

A greater length of fiber is required.
However, the transmission fiber in a telecom system may be used, so that no additional fiber is required.

Raman fiber amplifiers can have a lower noise figure.
On the other hand, they more directly couple pump noise to the signal than laser amplifiers do.

They also have a fast reaction to changes of the pump power, particularly for co-propagating pump, and very different saturation characteristics.

If the pump wavelength is polarized, the Raman gain is polarization-dependent.
This effect is often unwanted, but can be suppressed e.g. by using two polarization-coupled pump diodes or a pump depolarizer.

Fibers used for Raman amplifiers are not doped with rare earth ions.
In principle, any ordinary single-mode fiber could be used, and in practice the transmission fibers themselves are often suitable (→ distributed amplifiers).
However, there are special fibers with increased Raman gain, resulting from certain dopants (e.g. germania) for enhanced Raman cross sections, or simply from a small effective mode area.
Such fibers are used for lumped Raman amplifiers, where a shorter piece of fiber is dedicated to amplification only.

Arbitrary Index Profiles

A fiber's index profile may be more complicated than just a circle:

Here, we "printed" some letters, translated this into an index profile and initial optical field, propagated the light over some distance and plotted the output field – all automated with a little script code.